Beneficial Pseudomonas spp. produce an array of antimicrobial secondary metabolites such as cyclic lipopeptides (CLPs). We investigated the capacity of CLP-producing Pseudomonas strains and their crude CLP extracts to control rice blast caused by Magnaporthe oryzae , both in a direct manner and via induced systemic resistance (ISR). In planta biocontrol assays showed that lokisin-, white line inducing principle (WLIP)-, entolysin- and N3-producing strains successfully induced resistance to M. oryzae VT5M1. Furthermore, crude extracts of lokisin, WLIP and entolysin gave similar ISR results when tested in planta . In contrast, a xantholysin-producing strain and crude extracts of N3, xantholysin and orfamide did not induce resistance against the rice blast disease. The role of WLIP in triggering ISR was further confirmed by using WLIP-deficient mutants. The severity of rice blast disease was significantly reduced when M. oryzae spores were pre-treated with crude extracts of N3, lokisin, WLIP, entolysin or orfamide prior to inoculation. In vitro microscopic assays further revealed the capacity of crude N3, lokisin, WLIP, entolysin, xantholysin and orfamide to significantly inhibit appressoria formation by M. oryzae . In addition, the lokisin and WLIP biosynthetic gene clusters in the producing strains are described. In short, our study demonstrates the biological activity of structurally diverse CLPs in the control of the rice blast disease caused by M. oryzae . Furthermore, we provide insight into the non-ribosomal peptide synthetase genes encoding the WLIP and lokisin biosynthetic machineries.

Gibberellin (GA) regulates various plant growth and developmental processes, but its role in pathogen attack, and especially nematode-plant interactions, still remains to be elucidated. An in-depth characterization of the role of GA in nematode infection was conducted using mutant lines of rice, chemical inhibitors, and phytohormone measurements. Our results showed that GA influences rice-Meloidogyne graminicola interactions in a concentration-dependent manner. Foliar spray of plants with a low concentration of gibberellic acid enhanced nematode infection. Biosynthetic and signaling mutants confirmed the importance of gibberellin for rice susceptibility to M. graminicola infection. Our study also demonstrates that GA signaling suppresses jasmonate (JA)-mediated defense against M. graminicola, and likewise the JA-induced defense against M. graminicola requires SLENDER RICE1 (SLR1)-mediated repression of the GA pathway. In contrast to observations from other plant-pathogen interactions, GA plays a dominant role over JA in determining susceptibility to M. graminicola in rice. This GA-induced nematode susceptibility was largely independent of auxin biosynthesis, but relied on auxin transport. In conclusion, we showed that GA-JA antagonistic crosstalk is at the forefront of the interaction between rice and M. graminicola, and SLR1 plays a central role in the JA-mediated defense response in rice against this nematode.

Magnaporthe oryzae (rice blast) and the root-knot nematode Meloidogyne graminicola are causing two of the most important pathogenic diseases jeopardizing rice production. Here, we show that root-knot nematode infestation on rice roots leads to important above-ground changes in plant immunity gene expression, which is correlated with significantly enhanced susceptibility to blast disease. A detailed metabolic analysis of oxidative stress responses and hormonal balances demonstrates that the above-ground tissues have a disturbed oxidative stress level, with accumulation of HO, as well as hormonal disturbances. Moreover, double infection experiments on an oxidative stress mutant and an auxin-deficient rice line indicate that the accumulation of auxin in the above-ground tissue is at least partly responsible for the blast-promoting effect of root-knot nematode infection.

Root-knot nematodes (Meloidogyne spp.) are highly evolved obligate parasites that threaten global food security. These parasites have a remarkable ability to establish elaborate feeding sites in roots, which are their only source of nutrients throughout their life cycle. A wide range of nematode effectors have been implicated in modulation of host pathways for feeding site development. Plants produce a diverse array of peptide hormones including plant peptides containing sulfated tyrosine (PSYs), which promote root growth via cell expansion and proliferation. A sulfated PSY-like peptide RaxX (required for activation of XA21 mediated immunity X) produced by the biotrophic bacterial pathogen (Xanthomonas oryzae pv. oryzae), has been previously shown to contribute to bacterial virulence. Here, we report the identification of genes from root-knot nematodes predicted to encode PSY-like peptides with high sequence similarity to both bacterial RaxX and plant PSYs. Synthetic peptides encoding predicted nematode PSY domains (MigPSYs) stimulate root growth in Arabidopsis. Transcript levels are highest early in the infection cycle. Down-regulation of MigPSY gene expression reduces root galling and egg production, suggesting that the MigPSYs serve as nematode virulence factors. Together these results indicate that nematodes and bacteria utilize similar sulfated peptides to hijack plant developmental signaling pathways to facilitate parasitism.

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